[ ESO ]
ISAAC Quality Control:
photometric zeropoints

CAL | HC | refs | QC
Trending & QC1
Data Packages
Data Management
QC links:
zeropoints | FOV gradients
FOV gradients
QC1 database (advanced users): browse | plot
   Click on CURRENT to see the current trending (Health Check).
   Click on HISTORY to see the historical evolution of the trending.

Photometric zeropoints are acquired in the SW-arm in most nights ISAAC is in operation. The pipeline recipe extracts the zeropoint (for SW-arm and filters J, Js, Ks and H. The zeropoint is corrected for atmospheric extinction) as its main product. The photometric zeropoint is the magnitude that corresponds to the count rate of 1 ADU/sec.

There are the following settings

  • SW-arm (Hawaii) filters: J, Js, H, Ks (taken regularly)
  • LW-arm (Aladdin) J+Block, J, Ks; L, NM (taken only on nights with science data of that setting)

The jitter offset sequence of the the five raw input frames (Non-chopping mode) is shown in the following image. The first raw frame shows the photometric standard star in the center of the FOV; the second frame shows the star in the upper right quadrant a.s.o..


top photometric zeropoints

QC1 parameters

parameter QC1 database: table, name procedure
photometric zeropoint isaac_zp, zpoint_X0 - subtract from each of the five raw observations the consecutive frame as sky. Derive from each of the eight difference frames the source flux, and take the median of the min-max rejected sub sample. Read magnitude to identified star in catalog and correct for extinction.The following atmospheric extinction coefficients are used: kappa(J)=0.09; kappa(Js)=0.05; kappa(H)=0.04; kappa(Ks)=0.06. The extinction coefficients are taken from Mason et al. (2007) (see page 97). All raw frames are flatfielded but the linearity is only corrected for the Aladdin detector.
rms of the photometric zeropoint isaac_zp, zpoint_RMS - rms of the photometric zeropoint


The following list gives reason to the most striking discontinuities on the long term trending of the photometric zeropoints.

  • 2000-01-01 (currently unknown)
  • 2001-03-27 instrument intervention
  • 2001-08-21 instrument intervention
  • 2001-10-14 instrument intervention
  • 2002-03-14 instrument intervention
  • 2002-11-24 M1 recoating
  • 2003-03-28 instrument intervention
  • 2004-01-26 instrument intervention
  • 2004-04-03 M1 recoating
  • 2007-06-26 to 2007-01-01: intervention new ZPs (J from 25.07 to 25.25, H from 24.68 to 24.78, Js from 24.84 to 24.95, Ks from 24.17 to 24.25)
  • 2007-03-29: the J+Block filter supported with the new pipeline upgrade


top field of view gradients

The Field Of View gradient is derived from flux differences of the five offset positions. The slope is given in normalized flux units per 512 pixel. A Y-SLOPE of -0.02 means that a photometric standard star produces at detector position Y=1024 pixel 2% less counts, than at the center of the detector (X=512, y=512); and at Y=1 pixel %2 more counts than in the center of array. The errors of gradients are given as well, since large errors indicate non-photometric conditions at the time of template execution. The FOV gradients are a spatially low resolution illumination check. More accurate FOV resolved zeropoints are obtained by the illumination recipe, that are taken according to the calibration plan only after interventions, but at least once a year.

QC1 parameters

parameter QC1 database: table, name procedure
flux gradient over the field of view in x-direction isaac_zp, qc_grad_x, qc_grad_dx - average the flux measured in position 2 and 4, average the flux measured in position 3 and 5. Apply linear regression to both averaged points to get qc_grad_x and qc_grad_x
flux gradient over the field of view in y-direction isaac_zp, qc_grad_y, qc_grad_dy - average the flux measured in position 2 and 3, average the flux measured in position 4 and 5. Apply linear regression to both averaged points to get qc_grad_y and qc_grad_dy